Method of applying a bond coating and a thermal barrier coating on a metal substrate, and related articles

ABSTRACT

A method for applying at least one bond coating on a surface of a metal-based substrate is described. A foil of the bond coating material is first attached to the substrate surface and then fused thereto, e.g., by brazing. The foil is often initially prepared by thermally spraying the bond coating material onto a removable support sheet, and then detaching the support sheet. Optionally, the foil may also include a thermal barrier coating applied over the bond coating. The substrate can be a turbine engine component.

This application is a Divisional application Ser. No. 09/411,222 filedOct. 4, 1999, now U.S. Pat. No. 6,387,527.

This invention was made with government support under Contract No.DEFC21-95MC311 76 awarded by DoE. The government may have certain rightsto the invention.

BACKGROUND OF THE INVENTION

This invention generally relates to bond coatings and thermal barriercoatings applied to metals, e.g., metal components used in turbineengines. In some specific embodiments, it relates to improved techniquesfor applying such coatings to surfaces where access is difficult.

Metal components are used in a wide variety of industrial applications,under a diverse set of operating conditions. In many cases, thecomponents are provided with coatings which impart variouscharacteristics, such as corrosion resistance, heat resistance,oxidation resistance, and wear resistance. As an example, the variouscomponents of turbine engines, which typically can withstand in-servicetemperatures in the range of about 1100° C.-1150° C., are often coatedwith thermal barrier coatings (TBC's), to effectively increase thetemperature at which they can operate.

Most TBC's are ceramic-based, e.g., based on a material like zirconia(zirconium oxide), which is usually chemically stabilized with anothermaterial such as yttria. For a jet engine, the coatings are applied tovarious superalloy surfaces, such as turbine blades and vanes, combustorliners, and combustor nozzles. Usually, the TBC ceramics are applied toan intervening bond coating (sometimes referred to as a “bond layer”,“bond coat”, or “bond coat layer”) which has been applied directly tothe surface of the metal part. The bond coating is often critical forimproving the adhesion between the metal substrate and the TBC.

The effectiveness of a TBC coating is often measured by the number ofthermal cycles it can withstand before it delaminates from the substratewhich it is protecting. In general, coating effectiveness decreases asthe exposure temperature is increased. The failure of a TBC is oftenattributed to weaknesses or defects related in some way to the bondcoating, e.g., the microstructure of the bond coating. TBC failure canalso result from deficiencies at the bond coat-substrate interface orthe bond coat-TBC interface.

The microstructure of the bond coating is often determined by its methodof deposition. The deposition technique is in turn often determined bythe requirements for the overlying protective coating. For example, manyTBC's usually require a very rough bond coat surface for effectiveadhesion to the substrate. An air plasma spray (APS) technique is oftenused to provide such a surface.

While the APS process has several advantages, it also results in aporous coating microstructure. Such a microstructure allows significantinternal oxidation of the bond coating. The oxidation of regions of thebond coating often reduces the concentration of aluminum in other bondcoat regions. This phenomenon can in turn result in the diffusion ofaluminum from an adjacent, aluminum-containing substrate, e.g., asuperalloy. The depletion of aluminum from a superalloy substrate isespecially severe when the component is used at the elevatedtemperatures described above. The loss of aluminum can be detrimental tothe integrity of superalloy components.

In a pending U.S. patent application of M. Borom, et al., Ser. No.09/385,544, now U.S. Pat. No. 6,165,628 problems associated with themicrostructure of porous bond coats are addressed. In one embodiment ofthe reference, a bi-layer is used to bond a TBC to a metal substrate.The bi-layer includes a dense, primary bond coating over the substrate,and a “spongy” secondary bond coating over the dense coating. Theprimary bond coating is usually applied by a vacuum plasma spray (VPS)or high velocity oxy-fuel (HVOF) technique. The spongy, secondary bondcoating is usually applied by an air plasma spray technique. The primarybond coating helps to protect the substrate from excessive oxidation.The secondary bond coating promotes adhesion between the primary coatingand the subsequently-applied TBC, while also acting as a strain-relieverbetween these two other coatings. The resulting TBC system exhibits highintegrity during exposure to high temperatures and frequent thermalcycles.

Clearly, the various thermal spray techniques mentioned above are quitesuitable for applying bond coatings to many substrates. However, theyare sometimes not effective for applying the coatings to regions of asubstrate which are somewhat inaccessible, since the spray equipment maybe too large and cumbersome for such regions. For example, it can bevery difficult to thermally spray a bond coating on a flange or othersurface of a turbine engine part. Moreover, the spray process, which mayinclude one or more masking steps, is sometimes very time-consuming. Itis often very difficult to carry out local repairs using this process.

Thus, new methods for efficiently applying bond coatings and TBC's toinaccessible regions of a substrate would be welcome in the art. Themethods should also be capable of providing a bond coatingmicrostructure which protects the substrate from excessive oxidation.The methods should result in bond coats which provide a desirable levelof adhesion between the substrate and a subsequently-applied TBC. Theoverall TBC should be effective in protecting components used in highperformance applications, e.g., superalloy parts exposed to hightemperatures and frequent thermal cycles. It would also be desirable ifthe methods were generally compatible with conventional applicationequipment, e.g., various plasma spray techniques.

SUMMARY OF THE INVENTION

One embodiment of this invention is a method for applying at least onebond coating to a surface of a metal-based substrate, comprising thefollowing steps:

(a) attaching a foil which comprises the bond coating to the substratesurface, and then

(b) fusing the foil to the substrate surface, so that the bond coatingadheres to the substrate.

The foil is often prepared by thermally spraying the bond coatingmaterial onto a removable support sheet. Exemplary thermal spraytechniques are vacuum plasma deposition (VPS), high velocity oxy-fuel(HVOF), and air plasma spray (APS). When the support sheet is removed,the free-standing foil of bond coating material remains.

The free-standing foil is typically fused to the substrate surface by abrazing or welding process. Various brazing techniques are possible. Asan example, a slurry of the braze composition can be applied to asurface of the foil, which is then attached to the substrate surface,with the braze composition contacting the substrate. The brazecomposition is then exposed to a suitable brazing temperature. Analternative technique involves applying the braze slurry to thesubstrate surface first. The foil is then attached to the slurry-coatedsubstrate, followed by brazing. As still another alternative, a greenbraze tape cart be used to attach the foil to the substrate surface,followed by brazing.

The bond coating usually comprises an alloy of the formula MCrAlY, whereM is selected from the group consisting of Fe, Ni, Co, and mixtures ofany of the foregoing. In some embodiments of the invention, the foil ismade from at least two bond coatings. For example, it can be based on adense, primary bond coating and a “spongy” secondary bond coating overthe dense coating, as further described below.

Moreover, other embodiments of this invention include the application ofa thermal barrier coating applied over the bond coating on the removablesupport sheet. The TBC is usually zirconia-based, and can be appliedover the bond coating by various techniques, such as a plasma sprayprocess. Thus, the free-standing foil in this embodiment would includeboth a bond coating (or multiple bond coatings) and the TBC.

Yet another embodiment of this invention includes a method for repairinga worn or damaged thermal barrier coating system applied over ametal-based substrate. The method includes the step of removing the wornor damaged system (i.e., including at least one bond coating and theTBC), followed by replacement of the coating system, using thefree-standing foil mentioned above. As described previously, the foil isusually cut to the desired shape and brazed to the substrate surface.

Still another embodiment of this invention relates to an article whichincludes a metal-based substrate, such as a superalloy component of aturbine engine. The article further includes a foil which comprises atleast one bond coating, fused to the substrate. The foil may alsoinclude a TBC. As mentioned previously, the foil is preferably fused tothe substrate by an intervening coat of braze material.

The invention and its various embodiments are more particularlydescribed in the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

Various types of bond coatings can be applied by the method of thepresent invention. The compositions for such layers are well-known inthe art. Very often, the bond coatings are formed of an MCrAlY material,where “M” can be various metals or combinations of metals, such as Fe,Ni, or Co. Some of the preferred alloys of this type have a broadcomposition (in weight percent) of about 17% to about 23% chromium;about 4% to about 13% aluminum; and about 0.1% to about 2% yttrium; withM constituting the balance. In some embodiments, M is a mixture ofnickel and cobalt, wherein the ratio of nickel to cobalt is in the rangeof about 10:90 to about 90:10, by weight.

As mentioned above, the bond coating material is used in the form of afoil, i.e., a thin sheet which is sometimes referred to as a “coatingpre-form”. The thickness of the foil depends in large part on thedesired thickness of the bond coating or bond coatings. In general, thetotal thickness of the foil is usually in the range of about 25 micronsto about 900 microns, and preferably, in the range of about 100 microns500 microns (in the absence of any TBC, and after surface finishing,e.g., grinding).

The bond coating foil can be made by a variety of techniques. Forexample, the bond coating material, usually in the form of a powder, canbe deposited onto a removable support sheet (typically metallic) as athin layer of metal. In some embodiments, the support sheet is actuallya removable substrate, e.g., a replica or duplicate of the “finalsubstrate” (the component requiring the bond coating). Thus, the supportsheet would contain all of the curvatures of the final part. Asdescribed below, several techniques can be used to subsequently detachthe foil from the support sheet. When the free-standing foil isdetached, it contains all of the curvatures of the support sheet.

Various thermal spray techniques are usually employed for the depositionof the bond coating powder onto the removable support sheet. Examplesinclude YES, HVOF, or APS. Other deposition techniques could be used aswell, such as sputtering or physical vapor deposition (PVD). As onespecific illustration, HVOF techniques are known in the art anddescribed, for example, in U.S. Pat. Nos. 5,508,097 and 5,527,591, bothincorporated herein by reference. HVOF is a continuous combustionprocess in which the powder is injected into the jet stream of a spraygun at very high speeds. Those of ordinary skill in the art are familiarwith various HVOF details, such as the selection of primary gasses,secondary gasses (if used), and cooling gasses; gas flow rates: powerlevels; coating particle size, and the like.

As another illustration, plasma spray techniques are also known in theart and described, for example, in the Kirk-Othmer Encyclopedia ofChemical Technology, 3rd Edition, Vol. 15, page 255, and referencesnoted therein. U.S. Pat. Nos. 5,332,598; 5,047,612; and 4,741,286 arealso instructive in regard to various aspects of plasma spraying, andare incorporated herein by reference. In general, the typical plasmaspray techniques involve the formation of a high-temperature plasma,which produces a thermal plume. The coating material, in the form of apowder, is fed into the plume. The powder particles melt in the plasmaand are accelerated toward the substrate being coated. (If the processis carried out in an air environment, it is often referred to as APS.)Those of ordinary skill in the plasma spray coating art are familiarwith various details which are relevant to applying the coating, e.g.,plasma spray parameters such as spray distances (gun-to-substrate);selection of the number of spray-passes; powder feed rate, torch power,plasma gas selection; and the like.

Information regarding the other deposition techniques (e.g., vacuumplasma deposition, sputtering, PVD, and the like) is also readilyavailable. Those of skill in the art will be able to select particularoperating conditions for using each of these techniques to deposit afoil of the bond coating material on the support sheet.

As mentioned previously, the foil may comprise at least two bondcoatings. As an example, it may include a dense, primary bond coating,and a secondary bond coating which has a “spongy” microstructure. Theuse of such a bi-layer under certain application conditions results in avery desirable combination of coating properties, as further describedherein. This type of bi-layer is also described in the aforementionedpending patent application of M. Borom et al, Ser. No. 09/385,544 nowU.S. Pat. No. 6,165,628 herein referred to as “the Borom application”,which is incorporated herein by reference.

The dense, primary bond coating may be applied to the removable supportsheet by various techniques, such as a vacuum plasma spray technique oran HVOF technique. Vacuum plasma systems are known in the art. They areoften powered by direct current, and the process is carried out in alow-pressure environment, e.g., at about 20 torr to about 60 torr, withgreatly reduced levels of oxygen. These parameters minimize the level ofoxide inclusion, because there is much less oxygen available foraccumulation onto the molten coating particles. A conventional vacuumplasma spray gun can be used, e.g., an EPI 03CA gun from Electroplasma(Sulzer-METCO, Inc.). Other details are provided in the aforementionedreferenced application of M. Borom et al. HVOF techniques were describedpreviously.

The dense, primary bond coating may be applied to the removable supportsheet by various techniques, such as a vacuum plasma spray technique oran HVOF technique. Vacuum plasma systems are known in the art. They areoften powered by direct current, and the process is carried out in alow-pressure environment, e.g., at about 20 torr to about 60 torr, withgreatly reduced levels of oxygen. These parameters minimize the level ofoxide inclusion, because there is much less oxygen available foraccumulation onto the molten coating particles. A conventional vacuumplasma spray gun can be used, e.g., an EPI 03CA gun from Electroplasma(Sulzer-METCO, Inc.). Other details are provided in the referencedapplication of M. Borom et al, Ser. No. 09/385,544 now U.S. Pat. No.6,165,628. HVOF techniques were described previously.

The thickness of the dense, primary bond coating will depend in part onthe conditions to which the final, coated article will be subjected, aswell as other factors such as the level of oxidation protection requiredfor the article. Usually, the thickness will be in the range of about100 microns to about 400 microns, and preferably, in the range of about200 microns to about 300 microns.

For the bi-layer-embodiment, the secondary bond coating often includesan open network of interconnected pores, as described in theaforementioned Borom application. The pores are generally locatedbetween layers of oxide which are entrained on particles of the bondcoating material. This microstructure (which could be described as“spongy”) is in direct contrast to the dense bond coatings which werethought to be desirable in the prior art.

The bond coating microstructure may be characterized by “line length”,i.e., the sum of the strings of entrained oxide (i.e., lengths ofporosity) in a given section of the coating. Such a measurement can beobtained by an image analysis of the section, as described theaforementioned Borom application. In preferred embodiments, themicrostructure of the spongy bond coating has at least about 225continuous strings of oxide greater than 25 microns in length, persquare millimeter of sample (viewed in cross-section), as measured by anoptical microscope, 1500× magnification. In some especially preferredembodiments, the bond coating microstructure will have at least about300 continuous oxide strings greater than 25 microns in length. (Incontrast, the dense primary bond coating usually has less than about 200continuous strings of oxide greater than 25 microns in length, persquare millimeter of sample). (As described in the Borom application,the extent of porosity is sometimes also related to the oxide inclusionlevel). The spongy, secondary bond coating is usually applied by athermal spray technique, such as the plasma spray process describedpreviously.

The thickness of the secondary bond coating will depend on variousfactors, such as the oxidation protection and corrosion protectiondesired for the component, as welt as material costs. The shape and sizeof the part may also be considered, since the thickness of the bondcoating should not exceed dimensional tolerances in general, thethickness wilt be in the range of about 50 microns to about 500 microns,and preferably, in the range of about 100 microns to about 400 microns.In especially preferred embodiments, the thickness will be in the rangeof about 200 microns to about 300 microns.

Other bi-layer bond coating systems are possible. As an example, aprimary bond coating of the MCrAlY-type could first be applied to aremovable support sheet. An aluminide or noble metal-aluminide material(e.g., platinum-aluminide) coating could then be applied over theprimary bond coating, as a secondary bond coating. Various techniquesare available for applying such materials, e.g., pack aluminiding. Inone preferred embodiment for a platinum-aluminide coating, the platinumis first electroplated onto the primary bond coating, i.e., onto a foilof the primary bond coating which is usually attached to the removablesupport sheet. Electroplating of this type is often carried out withconventional P-salt or Q-salt electroplating solutions. In the secondstep, the platinum layer is diffusion-treated with aluminum vapor toform platinum aluminide. This exemplary foil thus includes a primarylayer of MCrAlY, and a secondary bond coating of platinum-aluminide.

As still another alternative, a three-layer bond coating system could beapplied over the removable support sheet. For example, a dense, primarybond coating could first be deposited, followed by the deposition of a“spongy” secondary bond coating, as described previously. An aluminideor noble metal-aluminide coating could then be applied over the spongy,secondary bond coating as a third coating. In such an instance, thislast-applied coating provides a measure of chemical resistance, whilethe two previously-deposited bond coatings perform the general functionsdescribed above.

After the bond coating or multiple bond coatings have been deposited,the support sheet is removed, leaving the desired, free-standing metalfoil. Several different techniques can be used to remove the foil fromthe support sheet. For example, if the support sheet is intentionallynot grit-blasted prior to deposition of the coating metal, adhesion ofthe metal to the support sheet will be relatively low, permitting easydetachment of the foil. Alternatively, a release coating can be appliedto the removable support sheet prior to application of the bond coatingmaterial. Suitable release coatings are known in the art and availablefrom Praxair, for example. As still another alternative, an etchablecoating such as aluminum can be applied to the removable support sheet,prior to application of the bond coating material. After the bondcoating material is applied, the coated support sheet can be treated ina bath of a solution which selectively etches the aluminum, e.g.,aqueous potassium hydroxide. Removal of the aluminum layer results indetachment of the foil from the removable support sheet.

In some instances, the substrate surface to which the foil will beattached is very curved or somewhat irregular. In such a case, it may bedesirable to provide the foil with a substantially identical shape.Relatively thin foils may be somewhat flexible, and can be bent to somedegree to match the curvature of a substrate. Foils of greater thicknessusually are not flexible, but can be shaped by other techniques. Forexample, the removable support sheet discussed above can initially beprovided with the desired curvature of the substrate, prior todeposition of the coating material. (If a replica of the final substrateis used as the support sheet, it would already have the desired shapeand curvature.)

Detachment of the removable support sheet results in a free-standingfoil of the bond coating material. (As described above, the foil couldbe in the form of a single bond coating or several bond coatings, e.g.,a spongy bond coating over a dense layer). The foil can then be cut to asize appropriate for the site on the substrate where the bond coating isdesired, prior to being fused to the substrate. Various techniques canbe used to temporarily hold the foil in place before fusing. Forexample, an adhesive could be used, i.e., one which completelyvolatilizes during the fusing step. Alternatively, the foil could bebolted, clamped, or tack-welded into place.

A variety of metals or metal alloys can be used as the substrate for thepresent invention. The term “metal-based” in reference to substratesdisclosed herein refers to those which are primarily formed of metal ormetal alloys, but which may also include some non-metallic components,e.g., ceramics, intermetallic phases, or intermediate phases. Usually,the substrate is a heat-resistant alloy, e.g., superalloys whichtypically have an operating temperature of up to about 1000-1150° C.(The term “superalloy” is usually intended to embrace complex cobalt- ornickel-based alloys which include one or more other elements such asaluminum, tungsten, molybdenum, titanium, and iron.) Superalloys aredescribed in various references, such as U.S. Pat. Nos. 5,399,313 and4,116,723, both incorporated herein by reference. High temperaturealloys are also generally described in Kirk-Othmer's Encyclopedia ofChemical Technology, 3rd Edition, Vol. 12, pp. 417-479 (1980), and Vol.15, pp. 787-800 (1981). Illustrative nickel-base superalloys aredesignated by the trade names Inconel®, Nimonic®, Rene® (e.g., Rene®80-, Rene® 95 alloys), and Udimet®. The type of substrate can varywidely, but it is often in the form of a jet engine part, such as aturbine nozzle.

The fusing step for attaching the foil of bond coating material to thesubstrate can be carried out by various techniques. Very often, it is abrazing step, and is similar to any conventional brazing operation. (Asused herein, “brazing” is generally meant to include any method ofjoining metals that involves the use of a filler metal or alloy.) Oneexemplary reference for details regarding brazing is the text ModernMetalworking, by J. R. Walker, The Goodheart-Willcox Co., Inc., 1965,pp. 29-1 to 30-24.

A variety of braze alloy compositions may be used for the presentinvention. Some of them are described in the Kirk-Othmer Encyclopedia ofChemical Technology, 3rd Edition, Vol. 21, pages 342 et seq. If thesubstrate is a nickel-base superalloy, the braze alloy usually containsat least about 40% by weight nickel. (Nickel-containing braze alloys orcobalt-containing braze alloys are usually used with cobalt-basesuperalloys). The braze alloy composition may also contain siliconand/or boron, which serve as melting point suppressants.

It should be noted that other types of braze alloys may be used, e.g.,precious metal compositions containing silver, gold, and/or palladium,in combination with other metals, sirch as copper, manganese, nickel,chrome, silicon, and boron. Mixtures which include at least one of thebraze alloy elements are also possible. Many of the metal brazecompositions are available from Praxair Surface Technologies, Inc.

Various techniques for applying the braze alloy can be employed. Forexample, the braze alloy composition can be applied to the removablesupport sheet, prior to application of the bond coating composition.Various thermal spray techniques can be used to apply the brazecomposition to the removable support sheet, such as HVOF and APS. Othertechniques can also be used, such as sputtering or PVD. When theremovable support sheet is detached, the braze layer will remainattached to the underside of the bond coating, i.e., forming adouble-layer (or “tri-layer” when two bond coatings are used) for fusingto the final substrate. In those Instances in which an etchable coatingis employed (as discussed above), the braze composition would be appliedafter the etchable coating is deposited. The solution employed to attackthe etchable coating should be one which will not adversely affect thebraze composition or any of the bond coating compositions.

In an alternative technique for applying the braze alloy, afree-standing braze foil could be employed. Methods for making suchbraze foils are known in the art. Moreover, the braze foils arecommercially available from various sources, such as Wesgo and AlliedSignal Company. The braze foil can be tack-welded to the substrate, oran adhesive can be used. The foil of bond coating material can then betack-welded or adhesively attached to the braze foil. Alternatively, thebraze foil can first be attached to the bond coating foil, followed bythe attachment of the joined foils to the substrate.

As still another alternative, a green braze tape could be used to attachthe bond coating foil to the substrate. Such tapes are well-known in theart, and are commercially available, e.g., the AMDRY® line of tapes fromSulzer-METCO, Inc. They can be obtained with an adhesive on one or bothsides, so that the tape can be, initially attached to either thesubstrate or the bond coating foil.

As another alternative, the braze material can be utilized in the formof a slurry which usually contains metal powder, binder, and optionally,solvent. A variety of binder materials may be used, e.g., water-basedorganic materials such as polyethylene oxide and various acrylics, orsolvent-based binders. Conventional details related to the mixing of theslurry are described in various references, such as U.S. Pat. No.4,325,754, which is incorporated herein by reference. Slurrycompositions are also commercially available. Use of the braze slurrycompositions is advantageous in various situations. For example, whenthe final substrate surface is irregular, or contains pits or crevices,the braze slurry can be used to fill such regions.

The braze slurry can be applied to the desired region of the finalsubstrate, prior to placement of the free-standing bond coating foilover the braze slurry. Various techniques are available for applying thebraze slurry composition. For example, it can be sprayed, painted, ortape-cast onto the final substrate. Alternatively, the braze slurrycomposition can be applied to the surface region of the foil which willcontact the desired region of the substrate. In fact, the braze slurrycomposition could be applied to both the bond coating foil and thesubstrate region which will be in contact with the foil.

Those of ordinary skill in the art are familiar with other detailsregarding brazing. Brazing temperatures depend in part on the type ofbraze alloy used, and are typically in the range of about 525° C. toabout 1650° C. In the case of nickel-based braze alloys, brazetemperatures are usually in the range of about 800° C. to about 1260° C.When possible, brazing is often carried out in a vacuum furnace. Theamount of vacuum will depend in part on the composition of the brazealloy. Usually, the vacuum will be in the range of about 10⁻¹ torr toabout 10⁻⁸ torr.

If the bond coating or multiple bond coatings are to be applied to anarea which does not lend itself to the use of a furnace (e.g., when thecomponent itself is too large to be inserted into a furnace), a torch orother localized heating means can be used. For example, a torch with anargon cover shield or flux could be directed at the brazing surface.Specific, illustrative types of heating techniques for this purposeinclude the use of gas welding torches (e.g., oxy-acetylene,oxyhydrogen, air-acetylene, air-hydrogen); RF welding; TIG (tungsteninert-gas) welding; electron-beam welding; resistance welding; and theuse of IR lamps. As described above, green braze materials usuallycontain organic binders which are volatile. Care should be taken whenusing these types of heating techniques with the green brazes, to avoidthe undesirable effects of out-gassing. For example, the heating stepcould be carried out very gradually. Moreover, one could select greentape compositions which have low volatile content. (Other alternativesare also possible, as described above, e.g., the use of metal brazefoils without binders).

As mentioned previously, the fusing step can be carried out bytechniques other than brazing. For example, a torch or other heatingtechnique (e.g., the welding techniques mentioned above) can be used forfusing the bond coating foil to the substrate, as an alternative to thevacuum furnace. Regardless of what fusing technique is employed, theresulting bond coating is metallurgically bonded to the substrate, andexhibits the properties of bond coatings applied by prior arttechniques.

The method described above can minimize or do away with time-consumingsteps often found in prior art processes. For example, masking stepsthat are usually required when coating a part (or regions of a part) canbe eliminated. Instead, the bond coating is formed “off-line”, as thefoil. The foil can be cut to precise dimensions and then brazed to theselected region of the part. In many instances, the brazing step can beadvantageously carried out during another heating step normally carriedout in the process. For example, the brazing step could be carried outduring a solution heat-treatment step, or during a TBC heat-treatmentstep. (Braze alloys having melting temperatures similar to temperaturesused in the other heating steps could be selected).

As mentioned previously, the foil used in the present invention canfurther include a thermal barrier coating, applied over the bond coating(or multiple bond coatings). Usually, the TBC is zirconia-based. As usedherein, “zirconia-based” embraces ceramic materials which contain atleast about 70% zircodia, by weight. Zirconia is a well-known compoundfor barrier coatings. Its use is described, for example, inKirk-Othmer's Encyclopedia of Chemical Technology, 3rd Edition, V. 24,pp. 882-883 (1984). In preferred embodiments, the zirconia is chemicallystabilized by being blended with a material such as yttrium oxide,calcium oxide, magnesium oxide, cerium oxide, scandium oxide, ormixtures of any of those materials. In one specific example, zirconiacan be blended with about 1% by weight to about 20% by weight yttriumoxide (based on their combined weight), and preferably, from about3%-10% yttrium oxide.

In some preferred embodiments, a plasma spray technique is employed toapply the TBC over the bond coating or multiple bond coatings. Suitableplasma-spray processes have been previously described herein, and in thevarious references. Again, those of ordinary skill in the art canselectively perform the routine preparation steps and adjust the variousprocess parameters, e.g., plasma spray distances; the number ofspray-passes; powder feed rate; powder particle size; and the like.Various other factors will be considered, such as the particularcomposition of the zirconia-based TBC, and the end use of the part beingcoated. The thickness of the TBC will depend in part on the particularcomponent being coated. Usually its thickness will be in the range ofabout 125 microns to about 2500 microns. In preferred embodiments forend uses such as airfoil components, the thickness is often in the rangeof about 250 microns to about 1150 microns.

The application of a TBC by this method also provides many advantagesover prior art processes. Again, masking steps can be eliminated becausethe entire TBC system—bond coating or multiple bond coatings with theTBC itself—is formed off-line as the foil. The foil can then be cut tothe specific, desired dimensions, and brazed to the selected region ofthe substrate.

Another embodiment of this invention is directed to a method forrepairing a worn or damaged TBC and bond coating (or set of bondcoatings) which have been applied over the substrate. Careful repair ofsuch layers is critical in preventing degradation of the substrate. Inthe case of a turbine engine component, for example, it may be necessaryto repair the coating while the turbine is in service, i.e., after itsdelivery from the manufacturing site. The process disclosed hereinprovides a means for rapidly repairing or replacing selected areas of anexisting TBC system (i.e., bond coating and thermal barrier coating),without having to completely remove the coatings from the entire part.The process is especially useful for repairing coatings which aresituated in areas not easily accessible to other repair techniques. Thesteps usually comprise:

(i) removing the worn or damaged bond coating and TBC from a selectedarea on the substrate;

(ii) attaching a foil which comprises the replacement bond-coating andTBC to the substrate surface, covering the selected area; and then

(iii) fusing the foil to the substrate, so that the bond coating and TBCadhere to the selected area on the substrate. (The bond coating-face ofthe foil is attached to the substrate, leaving the TBC-face exposed.)

The fusing step for this embodiment is often carried out by using atorch or other portable heating apparatus. In alternative embodiments,any of the individual bond coatings or TBC coatings could be depositedusing the foil-attachment technique, while the other coatings aredeposited in a conventional manner, e.g., thermally sprayed directly onthe substrate.

Yet another aspect of this invention is directed to an article whichcomprises a foil of bond coating material fused to a metal-basedsubstrate. The bond coating may include one layer or multiple layers, asdescribed previously. The substrate can be formed of various materials,such as superalloys, and is often in the torn of a turbine enginecomponent. The foil is usually fused to the substrate by an interveningbraze layer, as described above. The braze layer usually has a thicknessof about 2.5 microns to about 125 microns, and is usually no greaterthan about 25

The thickness of the foil (in this embodiment) will depend on thedesired thickness of the bond coating or multiple bond coatings.Usually, the thickness is in the range of about 25 microns to about 900microns, and preferably, in the range of about 100 microns to about 500microns, after surface finishing. When the foil is fused to thesubstrate (i.e., a turbine component), it functions as a bond coating,protecting the designated portion of the substrate, as describedpreviously.

It should be apparent from the preceding description that an additionalembodiment is directed to an article which comprises a metal-ceramicfoil of at least one bond coating and an overlying TBC, fused to ametal-based substrate. The foil is usually fused to the substrate by anintervening braze layer, as described previously. The TBC is oftenformed of a zirconia-based material, which is chemically stabilized asmentioned above. The thickness of the TBC-portion of the foil willdepend on the desired thickness of the TBC layer itself, for theparticular component being protected. Usually its thickness will be inthe range of about 125 microns to about 2500 microns. In preferredembodiments for end uses such as airfoil components, the thickness isoften in the range of about 250 microns to about 1150 microns.

The following examples are provided for illustration, and should not beconsidered to be any type of limitation on the scope of the presentinvention.

EXAMPLE 1

A gas turbine shroud formed of a nickel-based superalloy served as thesubstrate for this experiment. Prior to being coated, the surface of theshroud was purposefully not grit blasted (i.e., in contrast to typicalprocesses), to minimize the adhesion of subsequently-applied coatings. ANiCrAlY-type bond coat was then air plasma-sprayed onto the surface theshroud. A thermal barrier coating (zirconia, with 8 wt. % by weightyttria) was then air plasma sprayed over the bond coat.

After the turbine part cooled, the deposited coating “popped” off as onecontinuous sheet, i.e., the foil. The free-standing foil contained theprecise curvatures present in the original part. The foil was then cutwith a water jet into individual test pieces (0.64 cm-wide strip; 2.54cm disks). The free-standing disks were then brazed to a nickel-basedsuperalloy substrate, using Amdry™ 100 green braze tape, available fromSulzer-METCO, Inc. (The tape was about 0.005 inch/0.013 cm thick).

The green braze tape was sandwiched between the free-standing foil andthe metal substrate, and then vacuum-brazed for 30 minutes at 2100° F.(1149° C.). Upon removal from the furnace, the foil-coating wascompletely brazed to the metal substrate. Furnace cycle testing (FCT)was then carried out, with 1 cycle representing 45 minutes at 2000° F.(1093° C.). The testing demonstrated superior furnace cycle life for TBCsystems applied in this manner, as compared to conventionally-appliedair plasma-sprayed TBC systems. In an experimental run, failure for thecomparative (conventionally-coated) sample occurred at about 360 FCTcycles. In marked contrast, the sample of the present invention did notfail until after 1700 FCT cycles.

EXAMPLE 2

A flat, cold-rolled steel plate was used in this example. The plate haddimensions of 22.9 cm×12.7 cm×0.15 cm. The plate was wire spray-coatedwith about 381 microns (0.015 inch) of aluminum, which would serve as anetchable release layer.

The plate was then air plasma sprayed with a NiCrAlY-type bond coat(0.010 inch/254 microns thickness). Deposition of the bond coat wasfollowed by the air plasma-spraying of a zirconia-yttria TBC (0.015inch/381 microns thickness). The plate is then cut into 1 inch (2.54 cm)disks.

One of the disks was immersed in a water bath of 50:50 wt. % potassiumhydroxide:deionized water. The sample was bubbling furiously at theedges after 5 minutes. The sample was removed and inspected after 2hours of etchant exposure. The bond coat/TBC was not detached from thesubstrate. However, an inspection of the sample with a stereo-microscoperevealed that approximately 0.25 inch (0.64 cm) of the intermediatealuminum layer had been removed.

The sample was returned to the potassium hydroxide/water bath, andremoved after another 6 hours. The bond coat/TBC had been freed from thesubstrate. The free-standing coating (foil) was then brazed to anickel-based superalloy substrate, as described in Example 1. Furnacecycle testing (FCT) was then carried out as in the previous example. Thetesting again demonstrated superior furnace cycle life, with failure notoccurring until after 1100 FCT cycles.

Preferred and exemplary embodiments have been described herein. However,other modifications of the invention may be apparent to those skilled inthe art from these teachings. Therefore, it is intended that all suchmodifications which fall within the true spirit and scope of thisinvention be secured by the appended claims.

All of the patents, U.S. patent applications, articles, and textsmentioned above are incorporated herein by reference.

What is claimed:
 1. A method for applying at least one bond coating to asurface of a metal-based substrate, comprising the following steps: (a)attaching a foil which comprises the bond coating to the substratesurface, and then (b) fusing the foil to the substrate surface, so thatthe bond coating adheres to the substrate.
 2. The method of claim 1,wherein the foil is prepared by thermally spraying bond coating materialonto a removable support sheet to form the foil followed by separationof the foil from the removable support sheet.
 3. The method of claim 2,wherein thermal spraying is carried out by a technique selected from thegroup consisting of vacuum plasma deposition, high velocity oxy-fuel andair plasma spray.
 4. The method of claim 1, wherein the foil is fused tothe substrate surface by a brazing or welding technique.
 5. The methodof claim 4, wherein the brazing technique is carried out by applying aslurry of the braze composition to a surface of the foil, attaching thefoil to the substrate surface so that the braze composition is incontact with the substrate surface, and then exposing the brazecomposition to a suitable brazing temperature.
 6. The method of claim 4,wherein the brazing technique is carried out by applying a slurry of thebraze composition to the substrate surface; then attaching the foil tothe substrate surface so that the foil is in contact with the brazecomposition; and then exposing the braze composition to a suitablebrazing temperature.
 7. The method of claim 4, wherein the foil is fusedto the substrate surface by a technique which comprises the followingsteps: (I) applying a layer of braze alloy material to a removablesupport sheet; (II) thermally spraying the bond coating material overthe layer of braze alloy material, to form a bi-layer of braze alloy andbond coating material; (III) separating the bi-layer from the removablesupport sheet; (IV) attaching the bi-layer to the substrate surface sothat braze alloy material is in contact with the substrate surface; andthen (V) exposing the braze composition to a suitable brazingtemperature.
 8. The method of claim 4, wherein the brazing technique iscarried out by attaching the foil which comprises the bond coatingmaterial to the substrate surface with a green braze tape, and thenexposing the green braze tape to a suitable brazing temperature.
 9. Themethod of claim 1, wherein the bond coating comprises an alloy of theformula MCrAlY, where M is selected from the group consisting of Fe, Ni,Co, and mixtures of any of the foregoing.
 10. The method of claim 1,wherein the foil comprises at least two bond coatings.
 11. The method ofclaim 10, wherein the two bond coatings comprise: (i) a dense, primarybond coating; and (ii) a secondary bond coating over the dense coating,having a microstructure which comprises an open network ofinterconnected pores.
 12. The method of claim 11, wherein the dense,primary bond coating and the secondary bond coating each comprise analloy of the formula MCrAlY, where M is selected from the groupconsisting of Fe, Ni, Co, and mixtures of any of the foregoing.
 13. Themethod of claim 11, wherein the dense, primary bond coating of component(i) is applied by a vacuum plasma spray technique or by a high velocityoxy-fuel (HVOF) technique.
 14. The method of claim 11, wherein thesecondary bond coating is applied by a thermal spray technique.
 15. Themethod of claim 14, wherein the thermal spray technique is a plasmaspray process.
 16. The method of claim 11, wherein the microstructure ofthe secondary bond coating has at least about 225 continuous strings ofoxide greater than 25 microns in length, per square millimeter ofsample, viewed in cross-section.
 17. The method of claim 10, wherein thetwo bond coatings comprise: (i) a dense, primary bond coating; and (ii)a secondary bond coating over the dense coating, comprising platinumaluminide or platinum-nickel-aluminide.
 18. The method of claim 10,wherein the foil comprises three bond coatings.
 19. The method of claim18, wherein the three bond coatings comprise: (i) a dense, primary bondcoating; (ii) a secondary bond coating over the dense coating, having amicrostructure which comprises an open network of interconnected pores;and (iii) a third bond coating over the secondary bond coating,comprising platinum aluminide or platinum-nickel-aluminide.
 20. Themethod of claim 1, wherein the foil further comprises a thermal barriercoating, applied over the bond coating.
 21. The method of claim 20,wherein the thermal barrier coating is zirconia-based.
 22. The method ofclaim 20, wherein the thermal barrier coating is applied by thermalspray technique.
 23. The method of claim 1, wherein the metal-basedsubstrate is a superalloy.
 24. The method of claim 23, wherein thesuperalloy is nickel-based.
 25. The method of claim 1, wherein themetal-based substrate is a component of a turbine engine.
 26. A methodfor applying a zirconia-based thermal barrier coating over a surface ofa superalloy substrate, comprising the following steps: (a) attaching afoil which comprises at least one bond coating and an overlyingzirconia-based thermal barrier coating to the substrate surface; andthen (b) brazing the foil to the substrate surface.
 27. The method ofclaim 26, wherein the foil comprises a dense, primary bond coating and aspongy secondary bond coating over the dense coating, said secondarybond coating having a microstructure which comprises an open network ofinterconnected pores, wherein each of said bond coatings is formed of analloy material comprising MCrAlY, where M is selected from the groupconsisting of Fe, Ni, Co, and mixtures of any of the foregoing.
 28. Amethod for repairing a worn or damaged thermal barrier coating systemapplied over a metal-based substrate, said method comprising thefollowing steps: (i) removing the worn or damaged thermal barriercoating system from a selected area on the substrate; (ii) attaching afoil which comprises the replacement thermal barrier coating system tothe substrate surface, covering the selected area; and then (iii) fusingthe foil to the substrate, so that the thermal barrier coating systemadheres to the selected area on the substrate, wherein the thermalbarrier coating system comprises at least one bond coating for contactwith the substrate, and an overlying thermal barrier coating appliedover the bond coating.
 29. The method of claim 28, wherein the fusingstep is carried out by a brazing technique.
 30. The method of claim 29,wherein the brazing technique is carried out by attaching the foil tothe substrate surface with a green braze tape, and then exposing thegreen braze tape to a suitable brazing temperature.
 31. The method ofclaim 29, wherein the brazing technique is carried out with a portableheating device.
 32. The method of claim 29, wherein the foil is preparedby applying the thermal barrier coating system to a removable supportsheet to form the foil, followed by separation of the foil from theremovable support sheet.